It has been a long standing goal of industry to reclaim the heat escaping up the stack from furnaces used both in production processes and in generating electricity. Since direct use of the flue gases is impractical because of their chemically active or toxic components, most devices attempting this goal have used gas to gas heat exchangers where the flue gases heat incoming air which can be used either as combustion air or for other purposes such as heating buildings. Because the mechanics of the combustion processes producing the flue gases often require a free flow of these gases, it has frequently been necessary to use an intermediate heat transfer system to remove the heat from the flue gases in order to prevent interference with flue gas flow. Use has been made of both banks of individual heat pipes and conventional liquid-filled tube heat exchangers to serve as the intermediate heat exchange system. U.S. Pat. No. 4,044,820 by Nobles is one of the more recent approaches to the problem. Nobles uses a closely coupled heat exchanger in which a single pass through the hot gas is immediately followed by a pass through the cold gas and this sequence is repeated several times throughout the path of the intermediate liquid. While the closely coupled heat exchanger is desirable for its effectiveness, the Nobles approach leaves several other problems unsolved.
One problem which has been under attack is that of the danger of cooling the hot gas to the point of condensing some of the corrosive chemicals in it and thereby damaging the apparatus. The general approach to this problem has been to heat the incoming air by some means, simply increasing the temperature to which the hot gas is ultimately cooled. One patent that shows this approach is the U.S. Pat. No. 2,854,220 by Vaughan. The problem with heating the incoming air is that it reduces the amount of heat recovered by the heat exchanger regardless of whether condensation is a problem or not.
A further problem with the prior art is the lack of accomodation for normal required maintenance and emergency repairs. In the Nobles system for instance, a leak anywhere in the system requires shut-down of the entire system and maintenance cannot be performed until the entire furnace is shut down. In many installations, this can mean a long period of operation without the use of the flue gas heat exchanger, simply because of some minor fault in the heat exchanger.
It is an object of the present invention to furnish a highly effective heat exchanger for flue gas-to-air heat transfer and for other similar applications. Another object of this invention is to furnish a gas-to-gas heat exchanger which uses a minimum of auxiliary power for pumping the intermediate fluid. A further object is to furnish a heat exchanger which is automatically controlled to prevent condensation of corrosive flue gas vapors. Other objects of this invention are to yield a heat exchanger for which a leak or mechanical breakdown does not require shutdown of the entire unit, and for which fluid inventory and costs are minimized.